The present invention relates to a technique for forming an oxide coating film by anodic oxidation of metal, especially aluminum or an aluminum alloy. More specifically, the present invention relates to a forming electrolyte for forming an oxide coating film on metal, a method for forming an oxide coating film that utilizes the forming electrolyte, a metal whose surface has an oxide coating film that is formed by using the forming electrolyte, and a method for manufacturing a metal wiring whose surface has an oxide coating film that is formed by using the forming electrolyte. The present invention can effectively be utilized, in particular, in the forming treatment of gate wiring of thin film transistor (TFT) devices of liquid crystal display panels, or wiring of integrated circuits.
Metals and alloys are utilized in various industrial applications using their characteristic properties. In particular, aluminum, aluminum alloys and the like are effectively used for wiring of TFT devices, integrated circuits and the like because of their low specific resistance. As for the wiring of these devices, it is necessary to form an insulation film on its surface in order not to cause short-circuit between the wiring and other wiring or electrodes.
As a method of forming an insulation film on the surfaces of aluminum, aluminum alloys and the like, there has been known a forming treatment by anodic oxidation. In this method, an oxide film is formed on a surface of aluminum, aluminum alloys or the like by electrochemically oxidizing the surface in a forming electrolyte. Because this method has a function of recovering defects resulting from uniformity of substrate, it is excellent in that it can easily form a dense and smooth oxide coating film. For this reason, the oxide film forming method by the forming treatment has been effectively utilized in the wiring production process of TFT devices or integrated circuits.
As the forming electrolyte used for the oxide coating film formation for aluminum, aluminum alloys etc., various compositions have been proposed so far. For example, JP-A-58-147069 (the code xe2x80x9cJP-Axe2x80x9d herein used means a Japanese patent unexamined publication [Kokai]) discloses use of aqueous solution of ammonium tartrate, and JP-A-63-164 discloses use of aqueous solutions of citric acid and sodium tartrate as the forming electrolyte. Further, JP-A-61-133662 discloses use of a forming electrolyte composed of 1:3 mixture of 1% aqueous solution of ammonium borate or 3% aqueous solution of tartaric acid and propylene glycol. JP-A-2-85826 discloses use of a forming electrolyte composed of 3% aqueous solution of tartaric acid, which is diluted with ethylene glycol or propylene glycol, and adjusted to around pH 7 with aqueous ammonia. In JP-A-6-216389, used is a forming electrolyte composed of 3:7 (volume ratio) mixture of aqueous solution of 1% ammonium tartrate, aqueous solution of 1% ammonium adipate, aqueous solution of 1% ammonium oxalate or 1% aqueous solution of ammonium citrate, and ethylene glycol. In JP-A-8-50304, used is a forming electrolyte composed of 9:1:10 mixture of 3% aqueous solution of tartaric acid, 15% acetic acid, and 3% ethylene glycol. In JP-A-8-286209, an aqueous solution of inorganic acid ammonium salt selected from ammonium tetraborate, ammonium pentaborate, and ammonium borate, and aqueous solution of organic acid ammonium salt selected from ammonium tartrate, ammonium citrate, ammonium adipate, ammonium phthalate, ammonium oxalate, ammonium salicylate and ammonium carbonate are used as the forming electrolyte.
While various forming electrolytes have been proposed so far as mentioned above, those forming electrolytes could not afford an oxide coating film of sufficient insulation property when metal, in particular, aluminum or an aluminum alloy, is anodically oxidized by using these forming electrolytes. Therefore, in order to prevent dielectric breakdown, it has been necessary to further form another insulation film on the formed oxide film. Especially in case of manufacturing of TFT devices, a thick SiN film is formed on the oxide film by CVD to compensate insulation. Since CVD is performed at an elevated temperature, there has been arisen a problem that needlelike minute projections called hillocks are produced on the aluminum-containing surface of the metal, grow and penetrate the gate insulation film to generate defects in display panels during that operation.
Moreover, the anodic oxidation using the conventional forming electrolytes also suffers a drawback of slow formation speed. In order to obtain a higher formation speed, the formation current density must be made higher exceeding a required level. Therefore, raising the throughput using the conventional forming electrolytes has suffered a certain bound.
On the other hand, it has been proposed to use alloys composed of aluminum added with silicon or copper as wiring materials in order to suppress hillock generation in integrated circuits. Gate wiring composed of these alloys whose surfaces have anodic oxide coatings can be also used for TFT-LCDs, and using aluminum added with rare earth elements (JP-A-7-45555, JP-A-8-250494, and JP-A-8-306693), or added with valve metals (JP-A-8-286209) has been tried recently.
In order to effectively suppress the hillock generation by using aluminum added with rare earth elements, however, a relatively large amount of rare earth elements needs to be added (T.Onishi, E. Iwamura, and K. Takagi, J.Vac.Sci.Technol. A, 15 (4), 2339 (1997)). When a large amount of rare earth elements is added, however, a specific resistance of the wiring becomes quite high compared with that of pure aluminum, and there is a negative effect that the rare earth element is moved into an oxide coating film during anodic oxidation, causing a harmful influence. As for these methods, it has also been proposed that a part or whole of rare elements in a state of solid solution is deposited as an intermetallic compound by heat treatment at 300xc2x0 C. or higher to lower a specific resistance, but a wiring having a specific resistance of 4 xcexcxcexa9xc2x7cm or lower favorably used in a large or high-definition TFT-LCD has not been obtained. Therefore, the hillock generation could not be effectively suppressed while good physical property was maintained.
Therefore, the present invention aimed at solving the problems of these conventional arts.
That is, an object of the present invention is to provide a novel forming electrolyte which can form an oxide coating film having sufficient insulation property for metals including aluminum and aluminum alloys. Another object of the present invention is to provide a forming electrolyte that can afford a high formation rate and raise the throughput of processes including anodic formation. A still further object of the present invention is to provide a forming electrolyte that can suppress the hillock generation on metal surface by using that forming electrolyte for anodic oxidation of metals such as aluminum and aluminum alloys.
The present invention also aimed at providing a method for forming an oxide film of good insulation property at a high throughput, a metal having a highly insulating oxide coating film in which the hillock generation is suppressed, and a method for manufacturing an aluminum containing metal wiring capable of suppressing the hillock generation effectively while maintaining a low specific resistance.
In order to achieve the foregoing objects, the present inventors earnestly conducted studies. As a result, we found that a combination of particular solute and solvent can provide a forming electrolyte for forming metal oxide coating film which can achieve the objects, and thus completed the present invention. That is, the present invention provides a forming electrolyte for forming a metal oxide coating film which comprises one or more kinds of solutes selected from the group consisting a salt of inorganic acid and salt of organic carboxylic acid dissolved in a solvent having an alcoholic hydroxyl group or aprotic organic solvent, provided that, when the solvent having an alcoholic hydroxyl group is selected, the salt of organic carboxylic acid is selected from a salt of aromatic carboxylic acid, a salt of aliphatic polycarboxylic acid having 3-5 carbon atoms and no hydroxyl groups, a salt of monohydroxy carboxylic acid having 2-5 carbon atoms, and a salt of amino acid.
As the inorganic acid used for the forming electrolyte of the present invention, an inorganic oxoacid is preferable. In particular, one or more compounds selected from the group consisting of boric acid, phosphoric acid, sulfuric acid, tungstic acid, molybdic acid, chromic acid, and vanadic acid are preferable. As the organic carboxylic acid used for the forming electrolyte of the present invention, one or more compounds selected from the group consisting of salicylic acid, phthalic acid, benzoic acid, xcex3-resorcylic acid, maleic acid, fumaric acid, itaconic acid, malonic acid, succinic acid, glutaric acid, dimethylmalonic acid, citraconic acid, lactic acid, malic acid, aspartic acid, and glutamic acid are preferable. Further, as the solvent having an alcoholic hydroxyl group used for the forming electrolyte of the present invention, ethylene glycol or propylene glycol is preferable, and the xcex3-butyrolactone or propylene carbonate is preferable as the aprotic organic solvent.
The present invention also provides a method for forming a metal oxide coating film which method comprises a step of performing anodic oxidation of metal in the aforementioned forming electrolyte. The metal to be subjected to the anodic oxidation includes, for example, metal wiring thin films patterned on substrates and the like, and those obtained by sputtering of aluminum or aluminum alloys containing a rare earth element such as Sc, Nd, and Gd can be exemplified.
The present invention further provides metal, especially an aluminum alloy, having an oxide film that is formed by the aforementioned film forming method on its surface.
The present invention still further provides a method for manufacturing a metal wiring including a step of forming an oxide coating film by anodic oxidation in a nonaqueous solution containing a salt of inorganic oxoacid or a salt of organic carboxylic acid on an aluminum wiring containing in a range of 0.01% by weight to 8% by weight of a rare earth element, an aluminum wiring containing a rare earth element and having a specific resistance of 10 xcexcxcexa9xc2x7cm or lower, an aluminum wiring containing a rare earth element and having a peak intensity ratio of Al (220) peak to Al (111) peak by X-ray diffraction, Al(220)/Al (111), of 0.01 to 10000, or an aluminum wiring containing a rare earth element and having an integration intensity ratio of Al (220) peak to Al (111) peak by X-ray diffraction, Al (220) Al (111), of 0.01 to 10000.